In order to determine the angular position of a rotating shaft, a resolver was typically connected to the shaft. A conventional transmitter resolver has a primary coil wound on its rotor to which a high frequency (e.g. 4 KHz) excitation signal is applied. These prior art resolvers require brushes or a rotating transformer to transfer electrical current to the rotating excitation coil. Two secondary coils are spaced at 90 degrees with respect to each other around the resolver's stator. As the rotor spins, a signal is induced from the excitation coil into each of the secondary stator coils producing a high frequency signal on each of the secondary coils. The movement of the rotor modulates the induced signal so that it has a sinusoidal envelope.
By demodulating the signals from the stator coils, two sinusoidal waveforms are produced which have phase angles that correspond to the angular position of the rotor. The amplitude of the signal from one coil represents the sine of the shaft angle. Specifically the positive peak in the demodulated output from this coil is produced when the rotor is at the 90 degree position and a negative peak occurs when the shaft is at 270 degrees. Because the other secondary coil is displaced 90 degrees around the stator from the first coil, the amplitude of its demodulated output signal corresponds to the cosine of the rotor angle.
U.S. Pat. No. 4,449,117 issued on May 15, 1984 to Stephen Fortescue shows an electronic circuit for receiving the sine and cosine output signals from a resolver and producing a digital output representing the angular position of the shaft. The system that determines the angular position from the sine and cosine signals also has circuitry for determining the velocity of the shaft.
U.S. Pat. No. 4,481,468 issued on Nov. 6, 1984 to Tadahiro Ono, et al. shows a system for determining shaft velocity using a two-phase resolver. Two square wave excitation signals are applied to the sine and cosine coils to produce an output signal on the rotor coil. A wave shaper produces a square wave having the same period and phase as the output voltage from the rotor coil of the resolver. Triangular waveforms are produced from this square wave which are differentiated to derive the velocity of the shaft.